1. U.S. Pat. No. 2,210,416 PA0 2. German Pat. No. 89 363 PA0 3. German Utility Model No. 1 975 540 PA0 4. U.S. Pat. No. 4,238,020
Document 1 discloses clutch assemblies each of which having the features mentioned in the preamble of claim 1. There, the reservoir chamber has a larger inside diameter than the pressure chamber. Upon rotation, the fluid forms a torus of fluid in the reservoir chamber and is delivered into the pressure chamber by a non-rotating scoop tube. Located in the radially outer region of the pressure chamber is a outlet opening; therethrough the fluid may return into the reservoir chamber. The quantity of returning fluid is always smaller than the delivery of the scoop tube. Thus, the pressure chamber is filled with fluid, as long as the scoop tube delivers fluid.
A problem exists with these known clutch assemblies in that they do not remain open long enough during the start-up process, because the scoop tube already fills the pressure chamber at an early time. Therefore, in FIG. 1 of Document 1, the following has been provided for: the scoop tube may be shifted axially from the exterior, whereby the outlet opening of the scoop tube may be closed. Thus, the moment when the clutch assembly is closed may be determined arbitrarily. However, additional devices allowing an intervention from the exterior are necessary. A further disadvantage resides in the following: as long as the scoop tube dips into the rotating torus of fluid and as long as it cannot yet deliver fluid because its outlet opening is closed, a relatively high loss of torque is caused.
The latter also applies to the embodiment as per FIG. 5 of Document 1. There, a valve is arranged at the outlet opening of the scoop tube, the valve initially being kept closed by the force of a spring. It opens only when, upon the attainment of a preselected speed, a sufficiently high fluid pressure has built up in the scoop tube. Thus, in that case, no intervention from the exterior is necessary to keep the clutch assembly open for a sufficiently long time. However, there again exists the disadvantage that the scoop tube causes a loss of torque for a certain time.
In FIGS. 2 to 4 of said document 1, there is shown an embodiment which attempts to overcome the noted disadvantage. For this purpose, however, the scoop tube must be swingably mounted in such a way that its scoop opening is out of the torus of fluid during the start-up process. Furthermore, a lug 96 must be arranged on the scoop tube, which lug always dips into the torus of fluid in order to swing the scoop tube into its operating position when a preselected rotational speed has been attained. A disadvantage of this mode of construction exists in that it is complicated and susceptible to trouble and that, also in this case, a loss of torque is caused.
An additional demand frequently made on clutches of this type consists in the following: If the clutch assembly rotates in closed condition and the speed falls again for some reason, it is required that the clutch assembly be quickly opened (disengaged). With the known clutch assemblies of said document 1, automatic quick opening is not possible. Only the clutch assembly of said FIG. 1 can be made to open quickly; however, this requires a mechanical adjustment from the exterior against the pressure of the fluid in the pressure chamber (page 2, right-hand column, lines 43-50).
Document 2 discloses a clutch assembly wherein the reservoir chamber is arranged in a zone which is located closer to the rotational axis of the clutch than the pressure chamber. A variably adjustable throttle valve is provided for interconnecting both chambers. In this case, a scoop tube is not required because the fluid flow from the reservoir chamber to the pressure chamber is caused by virtue of centrifugal force. However, a major disadvantage is that when the clutch assembly is at rest, the pressure chamber can never be completely emptied. Consequently, it is to be expected that the clutch assembly, during a start-up process, generally closes much too early. This is true even in consideration of the fact that it takes a certain time at the beginning of the start-up process until the fluid contents of the pressure chamber have reached the rotational speed of the clutch half which encloses the pressure chamber. A further disadvantage is that a pressure force dependent on the rotational speed is exercised on the axially movable actuator also from the fluid reservoir. Thereby, the tendency to premature closing of the clutch assembly is further intensified.
Document 3 discloses a clutch assembly which serves for bridging a hydrodynamic coupling. There, the clutch assembly is enveloped by a shell or housing which rotates with one half of the clutch assembly. The shell is provided with an aperture concentric with the rotational axis of the clutch assembly. Located in the interior of the shell is the piston which rotates with said one half of the clutch assembly and is guided to be axially movable. The space located between the shell and the piston constitutes the pressure chamber which receives the actuating fluid. When the shell is rotating, a centrifugally induced pressure develops, which is dependent upon the rotational speed. This pressure again acts upon the piston, which hereby engages the clutch assembly against the force of a spring.
A conduit having a control valve extends from outside of the clutch assembly and through the said central aperture of the shell for filling the said pressure chamber. A quick drain valve is provided in the radially outer region of the shell to permit the draining of the pressure chamber in order to achieve clutch disengagement. A disadvantage of that known clutch assembly is that the fluid flowing out of the quick drain valve must be caught outside of the clutch assembly and must again (usually by a pump) be returned to the said pressure chamber. Further, the engaging and disengaging of the clutch assembly can be actuated only by an external control command, namely by actuation of the said control valve.